When I bought my first umbrella adapter I was looking for some of the cheapest models and ended up with the standard Impact model. Since then, I decided that I wanted to try a few different models... and now have the following (all of these have an angled receptacle for modifiers):
I have uses for each of them. Though, I don't think I would put any LARGE modifier and/or studio strobe on either of the Impact models, they work well for Speedlites and small/medium modifiers... and they are a bit cheaper than the Manfrotto. I was expecting something nice when I bought the Manfrotto but was fairly surprised at its build quality when I received it - it's built like a tank!
I also have the Westcott model that came with the Apollo Speedlight Kit. It does have a non-angled receptacle, but I think it's actually the best model for the Westcott. I think so primarily because the diameter of the stud that threads into the receptacle for the Apollo - I believe it's larger than any of the others, which does a better job holding the Apollo in place and keeping it from rotating/turning (which has a round shaft vs. the hex shaped ones that most umbrellas do).